A plurality of multimedia units connected to a management center receive and process broadcast data streams according to access rights stored in one or more security modules associated to these units. The latter comprise personal computers, decoders or mobile equipments.
A security module is a well-known tamper-proof device containing different encryption/decryption keys, information for identifying a user on a network and data defining rights acquired by the user for receiving a content of broadcast services. The security module can have different forms such as a removable smart card inserted in a reader, an integrated circuit welded on a mother board, a card of a SIM (Subscriber Identity Module) type present in the most of mobile equipments.
A multimedia unit includes a hardware and software central module of access control called CAK (Control Access Kernel) linked to the security module, to a module for demultiplexing the stream, to a decryption module, to applications such as an electronic program guide (EPG) and to a return channel towards a management center. The module CAK is in charge of, among other functions, forwarding, towards the security module, the control messages ECM and the management messages EMM extracted from the demultiplexed stream of digital data. The security module processes, verifies and decrypts the control messages ECM according to the rights stored and updated by the management messages EMM. The control words, extracted from the control messages ECM after their positive verification, are then transmitted to the decryption module in order to generate data in clear ready to be exploited by the user.
The streams received by the multimedia units transport a more and more important number of multiplexed channels, corresponding for example, to television programs and to different information services. The user selects these different channels by means of a program guide displayed on a screen in order to view them in live or in differed mode by recording them in a memory associated to the multimedia unit. This large number of programs and their operating possibilities (copies, temporary recording, and transfer to other units) also involve the storage of many rights in the security module. When the user selects a channel and then passes rapidly to one or several other channels, the processing time of data by the security module and the access control module CAK of the multimedia unit increases in line with the diversity of the access rights. In fact, after each selection of a channel the access conditions specific to each program or kind of program are compared with rights stored in the security module and when they become numerous, their processing can take several hundreds of milliseconds. This relatively long processing time can cause undesirable interruption of picture or sound during a fast passing from one channel to another, more commonly called zapping.
During the reception of television programs, the switching time between channels is due essentially to the acquisition of digital data comprising data necessary for decrypting and processing data of the new selected channel. Audio/video data are generally broadcast in a compressed form as MPEG format (Moving Pictures Expert Group). This format is divided according to a layer hierarchy in order to make easier error management, random search, editing, and synchronization with other signals, like the audio stream. The first layer called the video sequence layer comprises information like the size of the picture frame, the sampling rate (bit rate) and the picture rate (frame rate). The second layer is the group of picture layers comprising one or more groups of picture frames including:                picture frames of the I-frame type containing data for constructing a complete picture using compression very similar to JPEG (Joint Photographic Experts Group) compression.        frames of the P-frame type representing the differences with respect to a previous picture        and frames of the B-frame type representing the predicted picture according to the previous picture and the next picture.        
The third layer or picture layer comprises the frame and frame size information. A video sequence of a group of picture layers is made from the I-frame frame received more recently and from subsequent P and B frames. The I-frame of the beginning of the sequence is the most critical since, when the user changes the channel after having transmitted an I-frame, he must wait for the next I-frame transmission in order to decode the next P and B frames. The I-frames are generally transmitted at a frequency lower or equal to two per second. In the majority of applications the sequence is also protected by the encryption of the binary stream during the transmission. For decryption, the control words of the ECM messages of the conditional access system are also necessary for decrypting the I, P and B frames. The time taken by these decryption operations is added to the channel switching time especially when control messages ECM are transmitted at low data throughput.
The document WO2006044547 discloses a method for increasing the channels switching speed based on storage of channels information according to predetermined channels selection rules. In an example of implementation, the channels information comprises at least an I type frame, a control message ECM and a control word. A module for managing the control words receives ECM messages from several channels and selects the control word to be memorized according to rules stored in an appropriated module. Similar rules of input selection determine the channels from which the I-frames must be stored. For example the decoder firstly processes the most frequently viewed channels, the favorite channels determined by the user or the most important ones determined by an operator, the channel or channels adjacent to the current viewed channel. These lists of priority channels can also be updated dynamically during a change of channel by the user and periodically during a change of ECM messages.
The document WO03098919A1 describes a method based on the prediction of channels that a user could select as for example channels consecutive to the current channel. For the predicted channels, the demultiplexer provides the control word stream and the last decrypted control word is stored. During selection of the predicted channel, the control word can be immediately made available to the decryption module in order to allow the user to have quick access to the content of the channel.